Raman spectroscopy based measurement system

ABSTRACT

A method and system are presented for use in measuring one or more characteristics of patterned structures. The method comprises: performing measurements on a patterned structure by illuminating the structure with exciting light to cause Raman scattering of one or more excited regions of the pattern structure, while applying a controlled change of at least temperature condition of the patterned structure, and detecting the Raman scattering, and generating corresponding measured data indicative of a temperature dependence of the detected Raman scattering; and analyzing the measured data and generating data indicative of spatial profile of one or more properties of the patterned structure.

TECHNOLOGICAL FIELD AND BACKGROUND

The present invention is in the field of optical measurement technique,and relates to optical system and method for measuring in patternedstructures. The invention is particularly useful in metrologyapplications, used for example in semiconductor industry.

For several decades, semiconductor technology has been progressing byshrinking the size of devices, leading to impressive improvements inperformance and cost. However as computing increasingly becomes mobile,embedded and even wearable, an additional requirement emerges forimprovement in power efficiency. Because electrical resistance isinversely proportional to the cross sectional area of a conductor, andpower consumption is proportional to the number of transistors on achip, the traditional approach of geometrical shrinking is quicklybecoming unable to reconcile these two demands.

A solution to this problem which is increasingly adopted by thesemiconductor industry lies in manipulating not only the dimensions, butalso the material properties of devices—both intrinsic properties suchas composition, and extrinsic such as applied stress. This has beenshown to produce a dramatic increase in the mobility of charge carriers,and thus allows the continued scaling of devices without prohibitiveincreases in power consumption.

This emerging focus on material engineering makes material orientedmetrology in general, and Raman Spectroscopy in particular, importanttools for process optimization and control. The main advantage of RamanSpectroscopy is its direct sensitivity to material properties likecomposition, stress/strain, crystallinity etc. Raman Spectroscopy worksby probing vibrational modes of the sample, and the measured spectrum isusually comprised of a discrete set of peaks corresponding to thefrequencies of these modes.

US 2016/0139065, assigned to the assignee of the present application,discloses the technique of measuring one or more parameters of a sample.This technique utilizes hybridization approach according to whichmeasurements providing information on the sample geometry (such as OCDand/or CD-SEM) are used in combination with Raman spectroscopymeasurements in order to stabilize and verify the stress and compositiondistribution.

GENERAL DESCRIPTION

As described above, integrated structures became more complex withrespect to a variety of materials and geometry (e.g. complexity ofpatterns and decrease in the dimensions of the pattern features).Therefore, there is a need in the art for a novel measurement techniquethat would desirably increase the amount of information about thestructure, e.g. for controlling the patterning process applied to thestructure.

The present invention takes advantage of Raman-based measurementtechnique recently developed and described in WO 2017/103934, assignedto the assignee of the present application. This technique utilizesdetection of a Raman spectrum obtained from a patterned structure undermeasurements using selected optical measurement scheme(s) each with apredetermined configuration of illuminating and/or collected lightconditions corresponding to the characteristic(s) to be measured; anddetermining distribution of Raman-contribution efficiency (RCE) withinthe structure to determine the characteristic(s) of the structure.

There is a need in the art for a novel metrology technique/tool capableof accurately measuring not only the average material properties, butalso the spatial profile/distribution of these properties within astructure. This is especially true considering that the structuregeometry itself can be viewed as the spatial distribution ofelectric/optic properties, so this capability can also be applicable todimensional metrology.

The present invention is based on the inventor's understanding of thecapabilities and limitations of Raman spectroscopy. More specifically,the Raman response of a specific volume element depends on the localproperties of the material, such as composition, strain, crystallinityetc. The spatial distribution of these properties depends mostimportantly on the geometry of the structure (i.e. which materialsoccupy which space), but sometimes also on the subtler variation ofproperties within a single functional part of a structure (e.g. thespatial distribution of stress in a multilayer structure). However, thecollected Raman signal integrates light coming from the entireilluminated volume, making the measurement of local properties a seriouschallenge for Raman spectroscopy, and in some cases even compromisingthe ability to measure average integrated properties.

The interpretation of Raman spectra can conceptually be divided intothree levels, as follows: The first level relates to spectral separationof different materials. The Raman scattering from distinct materials,even if excited simultaneously, will generally have different spectraldistributions which can be identified by someone trained in the art andseparately quantified (e.g. by identifying peaks at specific locationsknown to belong to specific materials). This provides a high levelseparation of the signals coming from different materials in thestructure, and can in some cases even allow for a quantitativeestimation of the relative quantities of these materials.

The second level of interpretation relates to dependence of the Ramanpeaks and other features from each material on the specific propertiesof that material, including chemical properties (e.g. alloy composition,doping/impurities), and/or structural properties (e.g. crystalorientation and defect density, polycrystalline grain size distributionetc.), as well as physical conditions (e.g. stress, temperature). Thisdependence can be expressed through Raman peaks' locations, widths andshape distortions. This level of interpretation provides more detailedinformation compared to the first, but is usually still limited toaverage values over regions containing the same or similar materials. Itrequires on the other hand more refined measurement capability (in termsof spectral resolution and signal-noise ratio) and a theoretical orempirical model to support the interpretation of the measured spectra.

The third level relates to combining the information from multiplemeasurements, which probe different spatial regions of the sample. Thiscan be achieved by varying any number of system parameters affecting thecoupling of light into and out of the sample; these may includeillumination wavelength, angle of incidence, polarization etc. Byselectively exciting different parts of the sample, informationregarding the spatial profile can be extracted, as will be describedbelow. This type of interpretation requires an even more elaborate modelwhich can predict the spatial distribution of sources contributing tothe spectrum for a given set of system parameters, and the coupling ofthose sources to the detector. This kind of analysis is referred to as“profiling”.

Among the different techniques mentioned above for Raman basedprofiling, using multiple source wavelengths is notable for reliablyproducing a marked effect on the excitation profile, while other methodsmay vary in effectiveness depending on the structure geometry. This isdue to the strong dependence of the absorption coefficient on wavelengthin the visible range for relevant materials, such as Si, Ge and others.However, due to the strict characteristics required of light sources forRaman spectroscopy—they need to be ultra-narrow band, have high beamquality and very stable—adding several such sources to a Raman systemsignificantly increases system complexity and cost, making itimpractical to have more than 2-3 wavelengths.

The present invention provides a novel approach for monitoring/measuringof spatial distribution/profile of various parameters/properties of astructure, and also a complex patterned structure, from detected Ramanscattering/signature of the structure. It should be understood that theterm “spatial profile” or “spatial distribution” relates not only to theproperty(ies) variation across the structure but also through thestructure, and relates to both the material properties' and geometricalproperties' distributions. This is of particular importance when dealingwith multi-layer patterned structures, in which different measurementplanes inside the structure may include different layer stacks and/ordifferent patterns, with respect to both materials and geometry of thepattern features.

The approach of the invention relies on the dependence of variousproperties of a structure on temperature conditions of the structure.Such properties include for example, optical absorption of materialcomposition, strain, stress, etc., which depend on temperatureconditions up to some extent in all materials. The invention utilizes aneffect of such temperature dependence of the structure's properties ontoRaman scattering of the structure, and apply model-based processing tothe detected Raman signatures at different temperature conditions toobtain information indicative of the spatial profile of one or more ofthe structure properties.

More specifically, the invention is used to apply the temperaturechanges to the structure under exciting illumination to controllablyvary the absorption properties of the structure, and as a result thepenetration of the source beam (exciting illumination) into thestructure, and thus obtain information indicative of the spatial profileof the Raman sources (i.e. locations where Raman response isoriginated). Accordingly, the invention is described hereinbelow withrespect to this specific application of the invention. It should,however, be understood that the principles of the invention areapplicable for profiling of one or more other properties of thestructure, provided that the optical properties dictating theelectromagnetic field distribution are temperature-dependent whichaffects the Raman scattering, e.g. strain, stress, etc. For example,considering a solid Silicon wafer with a z-dependent strain, thetechnique of the invention can be used to separate the strain atdifferent depths, by producing measurements with different penetrationdepths. To this end, one or more appropriate models may be useddescribing Raman scattering from a structure, where the requiredproperty(ies) and temperature are the model parameters.

Thus, the measurement technique of the invention is based on performingRaman spectroscopy under controllably varying temperature conditions ofthe structure. Considering a change in the penetration depth of theexciting illumination as a function of temperature change, this resultsin exciting different Raman sources in the structure. Analysis of thedetected Raman responses of the excited regions/sources provides dataindicative of the spatial profile of the patterned structure beingmeasured.

According to one broad aspect of the invention, it provides a method foruse in measuring one or more characteristics of patterned structures.The method comprises: performing measurements on a patterned structureby illuminating the structure with exciting light to cause Ramanscattering of one or more excited regions of the pattern structure,while applying a controlled change of at least temperature condition ofthe patterned structure, and detecting the Raman scattering, andgenerating corresponding measured data indicative of a temperaturedependence of the detected Raman scattering; and analyzing the measureddata and generating data indicative of spatial profile of one or moreproperties of the patterned structure.

The controlled change of the temperature condition is implemented bycontrollably heating the pattern structure under illumination to affectcorresponding change in optical absorption of materials of the patternedstructure, e.g. local heating to affect the absorption change within theregion(s) being illuminated. By this, a change of one or more propertiesof the structure is affected, e.g. a penetration depth of the excitingillumination is affected, and the measured data is therefore indicativeof the excited/responding region(s) in the structure. Considering thepatterned structure, and even more a complex patterned structure, achange of the absorption properties, and accordingly a change in thepenetration depth, results in different Raman excitations causingvariation of the Raman scattering.

The heating field may be applied via the intensity of the excitingillumination causing the Raman scattering, and/or auxiliary illuminationapplied to the excited region(s).

The entire structure may be measured by performing a plurality ofmeasurement sessions on multiple measurement sites across the structure,e.g. selected sites. Generally, a number m of measurements (m≥2) isperformed on n measurement sites (n≥1) using two or more differenttemperature conditions. The plurality of measurements may furtherinclude controllable variation of wavelengths of the illumination and/orangles of incidence of the illumination and/or polarization states ofthe illumination. As described above, the measured data is thus in theform of temperature dependent Raman responses of the structure, which isat times referred to herein below as “temperature profile of Ramanscattering”.

The controlled change of the temperature condition is preferablyperformed by monitoring the temperature of the structure/region underillumination. This may be implemented by analyzing spectral informationin the temperature-dependent detected Raman scattering (e.g. using theprinciples of Raman thermometry).

According to another aspect of the invention, there is provided a systemfor measuring one or more characteristics of a patterned structure, thesystem comprising: a measurement system comprising: an illumination unitconfigured and operable to illuminate the patterned structure to causeRaman scattering of one or more excited regions of the patternedstructure and to cause controllable variation of at least a temperaturecondition of the structure; and a detection unit comprising at least oneoptical detector configured and operable to collect the Raman scatteringof the structure while under the controllably variable temperatureconditions, and generate corresponding measured data; a control systemconfigured to be in data communication with the measurement system, thecontrol system comprising an analyzer configured and operable to receivethe measured data and utilize data indicative of variation of thetemperature condition, and apply to the measured data model-basedprocessing and generate data indicative of a spatial profile of at leastone property of the patterned structure.

As described above, such at least one property of the structure includesat least material property, and preferably also includes geometricprofile of the structure.

The measurement system includes a light source system. In someembodiments, the light source system is configured and operable toproduce an illuminating beam which has wavelength and intensityproperties to cause both the Raman scattering and the controllablevariation of the temperature condition of the structure, to therebyaffect a penetration depth of the illuminating beam into the structureand excite the one or more regions to produce the Raman scattering. Insome other embodiments, the light source system is configured andoperable to produce separate light beams: an illuminating beamconfigured to excite the patterned structure to cause the Ramanscattering and a heating light beam configured to cause the controllablevariation of the temperature condition of the structure, to therebyaffect a penetration depth of the illuminating beam into the structureand excite said one or more regions to produce the Raman scattering.

The illumination causing the Raman scattering is preferably continuouswave illumination.

The measurement system may be is configured and operable to vary one ormore of the following measurement conditions: wavelengths of theillumination; angles of incidence of the illumination; and polarizationstates of the illumination.

The control system may be configured and operable to analyze themeasured data and determine the data indicative of variation of thetemperature condition from variation of one or more parameters in theRaman scattering data (e.g. peak intensity and/or peak spectral datasuch as wavelength and peak width).

The control system may also include and analyzer configured and operableto analyze the measured data by applying to the measured data one ormore predetermined models describing Raman scattering from a structure,where temperature and the one or more selected property of the structurearc included in a set of model parameters.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to better understand the subject matter that is disclosedherein and to exemplify how it may be carried out in practice,embodiments will now be described, by way of non-limiting example only,with reference to the accompanying drawings, in which:

FIG. 1 is a block diagram of a system of the invention for measuringcharacteristics of patterned structure;

FIG. 2 exemplifies an optical scheme suitable for use in the system ofFIG. 1;

FIG. 3 exemplifies dependence of the penetration depth of radiationthrough a material on the temperature conditions of the material;

FIGS. 4A to 4E exemplify how the technique of the invention can be usedfor determining two-dimensional spatial profile of the structure fromtemperature profile of the Raman response, where FIG. 4A exemplifies aportion (e.g. measurement site) of a multi-layer patterned structure,FIGS. 4B and 4D show more specifically two different regions of thestructure of FIG. 4A being measured, and FIGS. 4C and 4E show how theRaman spectra measured from the regions of FIGS. 4B and 4D,respectively, are modified with the temperature changes applied to thestructure (or at least those regions thereof); and

FIG. 5 exemplifies a flow diagram of a method of the invention formeasuring structure parameters.

DETAILED DESCRIPTION OF EMBODIMENTS

The present invention provides a system for monitoring characteristicsof patterned structures using Raman scattering from the structure whilevarying temperature conditions of the structure, to obtain temperatureprofile of the Raman signatures.

Referring to FIG. 1 there is illustrate, by way of block diagram, asystem 100 according to the invention. The system 100 includes ameasurement system/unit 102 and a control system 104 which areconfigured for data/signal communication between them (via wires orwireless signal connection using any known suitable communicationequipment and protocols).

The measurement system 102 is appropriately mounted with respect to astage 105, which defines a measurement plane MP for supporting astructure 10 under measurements, e.g. semiconductor wafer. The stage 105typically has a drive unit 107 which has a mechanism configured andoperable to adjust the z-position of the measurement plane with respectto the measurement system, and may also include mechanism(s) for linearmovement along either one or both of X- and Y-axes and/or rotation inthe X-Y-plane.

The measurement system 102 includes an illumination system 106 and adetection system 108, and a light focusing and collecting optics, aswill be described below. The illumination system 106 includes a lightsource 106A (e.g. laser(s)) configured and operable to illuminate thepatterned structure with exciting wavelengths to cause Raman scatteringfrom one or more excited regions. Further, the illumination system 106is preferably configured to illuminate at least the regions beingexcited by heating illumination, in order to cause controllablevariation of a temperature condition of those regions.

It should be understood that, generally, for the purposes of the presentinvention, the entire structure under Raman-based measurements may besubject to controllably variable temperature conditions. Preferably,however, local heating of the regions being measured is used, by meansof applied radiation. Further, as shown in the figure by dashed lines,an auxiliary heating light source 106B might be used. Alternatively oradditionally, the local heating effect can be achieved by the sameexciting beam produced by light source 106A which thus causes both thecontrollable change of the local temperature and corresponding Ramanscattering. It should also be noted that different temperatureconditions may be performed by continuously changing the temperature, orapplying two or more different temperatures, as the case may be.

The measured data is thus in the form of temperature dependent Ramanresponses of the structure, which is at times referred to herein belowas “temperature profile of Raman scattering”. This will be exemplifiedmore specifically further below.

The detection unit 108 includes one or more detectors (spectrometer(s))configured and operable to receive the Raman scattering light andgenerate measured data MD indicative thereof. As described above, theRaman scattering is being detected during the controllably variabletemperature conditions, e.g. two or more different temperatures.Accordingly, measured data MD is indicative of a temperature profile ofthe Raman scattering, i.e. is a function of Raman response wavelength(s)λ and temperature t°. As will be exemplified below, the temperaturechange affects absorption coefficient of the material, and accordinglyaffects a penetration depth of the exciting radiation, and thus thelocation at which the Raman response is originated. Therefore, suchmeasured data) MD(λ,t°) provides a direct measure of the spatialdistribution/profile of one or more properties of the structure,including both the material and geometric properties of the structure.Using model-based processing of such measured data provides fordetermining the parameters' distribution within the structure.

The system 100 is configured and operable to perform one or moremeasurement sessions on the patterned structure 10 to obtain dataindicative of the spatial profile of the patterned structure 10. Eachmeasurement session includes illuminating a region (focal spot) IR onthe patterned structure 10 by continuous wave illuminating beam(s), IB,of exciting wavelength(s), while being concurrently heated byillumination to cause temperature changes within the region IR, tothereby cause Raman scattering/response(s) RB from differently locatedRaman sources within the illuminated region. For example, increase oftemperature decreases the absorption coefficient for a certainwavelengths and thus decreases the penetration depth of the excitingillumination. Accordingly a different pattern inside the structure isbeing excited to cause the Raman response.

As described above, such heating may be achieved by the same excitingbeam IB and/or by a separate heating beam HB whose intensity(ies) is/areappropriately controllably varied by the temperature controller 104D. Itshould be understood that in case the separate (auxiliary) heating lightbeam is used, the heating beam wavelength may be outside spectralrange(s) used for Raman excitation, and may be continuous wave or pulsedbeam. The controlled change of the temperature condition may be achievedby controlled variation of the intensity of the illuminating beam,either beam IB or beam HB or both of them, as the case may be, tothereby vary a heating field applied to the region being illuminated.

Such a temperature change (heating) results in a corresponding change inoptical absorption of the illuminated region due to the strongdependence of the absorption coefficient of a medium on the temperatureof said medium. Generally, the optical absorption of a medium affectslight penetration depth into the medium. Thus, a change in thetemperature condition causes a change in the optical absorption whichresults in a different location of the Raman source excited by theillumination (e.g. deeper location at lower temperature). Hence, theRaman responses (wavelengths) detected from multiple focal spots acrossthe structure (by scanning) and the corresponding temperatures formtogether a spatial profile of the Raman scattering, which can beinterpreted as the spatial profile of the source(s) of the Ramanscattering.

The control system 104 includes data input and output utilities/modules104A and 104B, memory utility 104C, and also includes a temperaturecontroller 104D and illumination controller 104E which controllablymodify the wavelength and temperature parameters/conditions during themeasurements. The temperature controller 104D is configured and operableto monitor the temperature conditions of the excited/responding regionsand provide respective temperature data TD enabling to associate thetemperature parameters with the detected Raman scattering. To this end,the temperature controller 104D may utilize any known suitabletemperature measurement techniques. For example, the principles of Ramanthermometer can be used.

One known method of Raman thermometer is based on the fact that thematerial's temperature can affect the peak position of Raman bands.Thus, when a Raman band shifts significantly with temperature, themonitoring of the peak position can be the most straightforward mannerof determining temperature provided the Raman spectrometer hassufficient spectral resolution. According to another method, thetemperature can be determined from a ratio of the Stokes and anti-Stokessignal strengths of a given Raman band. For the latter method, thedetector is capable of detecting light at wavelengths longer and shorterthan that of the exciting light source (laser).

Thus, if a Raman band is sufficiently narrow, the peak position(wavelength) can be seen to shift with the temperature. As thetemperature increases, the bond length increases and consequently theenergy of the vibrational mode decreases; a decrease in temperatureleads to a shorter bond length and an increase in the energy of thevibrational mode. The increase or decrease in bond length causes achange in the vibrational force constant, which results in a shift ofthe Raman peak position. An other way of determining the temperature isby measuring the signal strengths of a particular Raman band at theStokes and anti-Stokes positions and calculate the temperature based ona Boltzmann distribution of the ground and first excited statepopulations, according to the relation

${\frac{I_{{anit} - {stokes}}}{I_{stokes}} = \left( \frac{\omega_{photon} + \omega_{phonon}}{\omega_{photon} - \omega_{phonon}} \right)^{4}}e^{- \frac{{\hslash\omega}_{phonon}}{kT}}$

Here, I_(stokes) and I_(anti-stokes) are measured intensities of Stokesand anti-Stokes photons in the Raman spectra, ω_(photon) and ω_(phonon)are the frequencies of the exciting light and of the excited Raman mode,and T is the temperature.

The Raman thermometry is generally known and does not form part of theinvention, and therefore need not be described in more details, exceptto note that for the purposes of the present invention, which is aimedat determining the spatial profile of Raman scattering from thetemperature profile of such scattering, the same Raman response can beappropriately analyzed to determine such information as the respondingmaterial, the corresponding temperature and the material location in thestructure, as will be described further below.

The control unit 104 may also include a scan controller 104F. In thisconnection, it should be noted that the entire structure can be measuredby using a scan mode, by providing a relative displacement between thestage 105 and an optical head (not shown here) of the measurement system102. This can be achieved by moving the stage 105 and/or at least someelements of the optical head. The stage 105 may be a so-called X-Y-stageor r,θ-stage. For example, considering the geometrically symmetricstructures (e.g. disk-like structures, as semiconductor wafers), ther,θ-stage can be used and the measurements are performed by scanning thefirst half of the structure using linear movement(s) of the stage and/oroptics, and then 180° rotation of the stage and repeat the scan of thesecond half of the structure. As described above, and illustrated in thefigure, the respective drive circuits/mechanisms of the stage and/ormeasurement system are appropriately connected to the scan controller104F of the control system 104.

The system 100 also includes a data analyzer 110, which may be a part(module) of the control system 104, or may be a separate device, as thecase may be. The data analyzer 110 is in data communication with thedetection unit 108 and with the illumination and temperature controllers104E and 104D to controllably operate the illumination and the heatingconditions (i.e. appropriately vary the exciting wavelengths andintensities, EI(λ₁, λ₂, λ₃)) and heating wavelength and intensity,HI(λ₄), during measurement sessions, and to receive themeasured/detected Raman scattering MD and the temperature data TD, andgenerate data indicative of a spatial profile of material properties ofthe patterned structure.

It should be understood that the technique of the invention is neitherlimited to a number of exciting wavelengths nor that of the heatingradiation, in case a separate heating light beam is used, and theexciting wavelengths λ₁, λ₂, λ₃ and heating wavelength λ₄ areexemplified here just in order to emphasize that multiple excitingwavelengths are used to excite various different Raman sources and thatthe wavelength of the heating beam, if used, may be different from theexciting wavelengths.

Reference is made to FIG. 2 which exemplifies an optical head 112 (or apart thereof) and a light propagation scheme suitable for use in themeasurement system 102. The optical head 112 includes at least anobjective lens 112B having an optical axis OA₂, which is located in acommon path of illumination and collection channels and focusesilluminating beams IB propagating from light source(s) onto an excitingregion ER and collects Raman scattering beams RB to be directed to thedetector 108.

As exemplified in the figure, the optical head 112 may also include alens 112A located in the optical path of illuminating beam IB upstreamof the objective 112B, and having a numerical aperture larger than thatof the objective. Moving the lens 112A along the X-axis with respect toobjective 112B within the field of view of objective 112B provides forangular scanning or angle resolve measurements (different angles ofincidence) of the illuminating beam onto the structure.

For example, considering a measurement session as corresponding to afixed relative position between the stage 105 and the objective 112B,multiple measurements with different angles of incidence can be takenfrom regions IR by moving the lens 112A along a distance correspondingto the lateral dimension of the objective 112B. Also, for example,during these multiple measurements or a single measurement within thesame measurement session (i.e. for the same illuminated region IR), thetemperature conditions and/or the wavelengths can vary, resulting in atemperature profile (temperature dependence) of Raman responses permeasurement session. Then, the relative movement between the stage andthe optical head is performed to implement the scan mode, and the nextmeasurement session is performed, during which lens 112A may or may notbe moved with respect to objective 112B, as described above. Themeasured data obtained/detected during the complete scan is thus afunction of wavelength and location across the structure (in theX-Y-plane), and is also a function of temperature, and thus providesmaterial information through the structure (along Z-axis), resulting ina spatial profile of Raman scattering (material-related and geometricalinformation).

Temperature dependent absorption properties n&k (refractive index &extinction coefficient) can be used to determine the penetration depth.For example, the inventor used such data for crystalline Silicon as arepresentative material [Vuye et al. (1993), “Temperature dependence ofthe dielectric function of silicon using in situ spectroscopicellipsometry”, Thin Solid Films 233, 166-170 (1993)] to calculate thepenetration depth as π⁻¹=λ/4πk. FIG. 3 illustrates optical penetrationdepth in Si vs. temperature for some typical wavelengths, 405 nm, 423nm, 458 nm, 488 nm, used in Raman Scattering. As can be seen,temperature changes going from room temperature to about 300-400 Ccauses an about two-fold change in the penetration depth.

With regard to a heat source, as described above, any way to heat thestructure under measurements can be used, such as placing the entirestructure in a temperature controlled enclosure. This has an advantageof producing a known uniform temperature throughout the measured device.However, it might not be well suited to a production environment wheremeasurement time and equipment cost are important. The inventiontherefore may utilize a different method using heat dissipation fromeither the Raman excitation beam itself, or an auxiliary laser beam at adifferent wavelength (e.g. in the near infra-red) which does notinterfere with the Raman signal and is only used for local heating ofthe sample. This approach, although it might result in that thetemperature profile is highly non-uniform and has to be modeled inaddition to the electromagnctics for the data to be successfullyinterpreted, is advantageous with regard to a fast response and finecontrol of the average heat flux over a wide range.

It should also be noted, although not specifically shown, that themeasurement system may include various other optical elements, includinglight directing elements, such as one or more of the following:re-directing mirrors, optical fibers, wavelength selective elements,e.g. dichroic mirror(s), beam splitters(s).

Also, as shown in FIG. 1, the measurement system 102 may includepolarizers P₁ and P₂ in the illumination and collection channels. Forexample, Raman scattering signal may be filtered by acquiring spectrawith polarization which is either parallel or perpendicular to thepolarization of the excitation light produced by polarizer P₁. To thisend, polarizer P₂ may be inserted in the beam path between themeasurement plane MP (structure) and the detector 108 (spectrometer),allowing the Raman polarization to be selected. The polarization of theilluminating beam can also be kept in its normal state, rotated by 90°,or ‘scrambled’ to remove any polarization by inserting polarizing opticsP₁ between the light source (laser) 106A and the structure. Polarizationmeasurements also provide useful information about molecular shape andthe orientation of molecules in ordered materials, such as crystals,polymers and liquid crystals.

Reference is now made to FIGS. 4A to 4E exemplifying how the principlesof the invention can be used for determining the two-dimensional spatialprofile of the structure (i.e. across and through the structure) from atemperature profile of the Raman response. Here. FIG. 4A exemplifies aportion (measurement site) 116 of a multi-layer patterned structure 10,such as a semiconductor wafer, which in the present not limiting exampleis formed by layers L₁, L₂, L₃ made of different materials 1, 2 and 3,respectively. Layer L₂ is located on top of substrate layer L₁, andlayer L₃ is located in top of layer L₂ and is patterned forming an arrayof spaced-apart regions of material 3. Measurements are exemplified asbeing performed on two regions of different types, i.e. having differentlayer stacks and/or different patterns. In the present example, oneregion 116A includes the line-feature of material 3 (FIG. 4B) and theother region 116B has no such feature, e.g. a space-feature of layer L₃pattern (FIG. 4D). FIGS. 4C and 4E show how the Raman spectra of thematerials included in the portions 116A and 116B, respectively, aremodified with the temperature changes.

Thus, region 116A includes layer L₁ (substrate) made of Material 1,layer L₂ of Material 2, and layer L₃ of Material 3, shown in FIG. 4B.Region 116B includes substrate layer L₁ of Material 1 and layer L₂ ofMaterial 2, as shown in FIG. 4D. The Raman response of a specific volumeelement within the portion 116A being measured depends on the localproperties of the materials 1, 2 and 3, such as composition, strain,crystallinity etc., which are effectively embodied in the Raman tensorsassigned to that volume. Similarly, the Raman response of portion/region116B depends on the local properties of the materials 1 and 2. Asdescribed above, the spatial distribution of these properties depends onthe geometry of the structure (i.e. which materials occupy which space),hut sometimes also on the subtler variation of properties within asingle functional part of a structure (e.g. the spatial distribution ofstress in an etched Si line structure). The collected Ramansignal/response is integrated over the entire illuminated volume, makingthe accurate measurement of local properties a serious challenge forRaman spectroscopy.

Each of FIGS. 4C and 4E shows two Raman responses, I₁(λ) and I₂(λ), ofthe volume/portion 116A/116B measured under different temperatureconditions T₁ and T₂, respectively. As seen in FIG. 4C, both graphs haveRaman peaks R₁, R₃ and R₂ corresponding to Materials 1, 2 and 3,respectively. Heating of the portion 116A from T₁ to T₂ temperaturecondition affects the entire Raman response/signature, while differentlyaffecting the Raman peaks of three materials: the spectralcharacteristics of the Raman peak of material 2 (i.e. wavelength andpeak width) remain almost unchanged hut intensity is slightly reduced,the Raman peak of material 1 shows significant reduction in theintensity and a detectable spectral shift with almost unchanged peakwidth, and the Raman peak of material 3 is changed, althoughinsignificantly, in all three parameters—spectral shift, peak width, andintensity. Such a change in the Raman signature mayexplained/interpreted as follows: at higher temperature T₂, thepenetration depth of the illuminating beam is lower, and accordingly theRaman response of substrate material 1 in the detected signal is lowerthan at the lower temperature T₂; the excitation of material 3 isoppositely affected by the heating; and that of material 2 is almostunaffected. As for the Raman signature measured on portion/volume 116Bshown in FIG. 4E, both graphs I₁(λ) and I₂(λ) have Raman peaks R₁ and R₂corresponding to Materials 1 and 2, respectively. Increase oftemperature from T₁ to T₂ results in the almost unchanged Raman peak R₂of material 2 with slightly increased intensity and significantlyincreased intensity of Raman peak R₁ of material 1 with some spectralshift of the peak. For both volumes 116A and 166B, the temperaturedifference Δt=(T₂−T₁) can be determined from the peak shift of material1, and plugged/injected into a Raman response model to obtain thecorresponding field distributions at different temperatures.

The following is an example of the interpretation of the detected Ramanspectra according to the above-described three levels of datainterpretation.

As described above, Raman scattering from distinct materials, even ifexcited simultaneously, will generally have very different spectraldistributions which can be identified and separately quantified. Thismakes a high level separation of the signals coming from differentmaterials in the structure, and can even allow an estimation of therelative volume of these materials across the structure. Also, asdescribed above, the Raman peaks and other features from each materialdepend on the specific properties of that material and physicalconditions (e.g. temperature, stress). This dependence can be expressedthrough peak locations, widths and shape distortions, as exemplified inFIGS. 4C and 4E.

Combining the information from multiple measurements, which probedifferent spatial regions of the structure is achieved by varying systemparameters affecting the interaction of light with the structure (e.g.coupling light into and out of the structure), such as wavelength, angleof incidence, polarization etc.

Let us consider the structure as being characterized by a discrete setof N parameters P_(i), such as various geometrical and materialquantities. In special cases this set can be decoupled such that smallsubset of measured parameters (e.g. the positions of only one or twopeaks) can be used to measure a corresponding subset of parameters (e.g.the composition of a specific material in the sample). The measured dataacquired by one Raman measurement can be described by some vector ofquantities, Y_(j), that can be raw spectral data, or fitted parametersextracted by model-based processing that data.

The measured values Y are related to the sample parameters P through anoperator

, such that Y=

P. Extracting the parameters can now be thought of as inverting theoperator

, which can be done using well known methods. To this end, the problemis to be well-posed (i.e. to have a higher number K of independentmeasurements than the number N of parameters, K>N), and the operator

is to be known. This requires modelling of light-matter interaction inorder to predict the electromagnetic field distribution inside thestructure. For the technique of the invention, thermal modelling is alsorequired to account for the temperature dependence of optical andelastic properties.

With regard to the “well-posedness” of the inverse problem, it shouldalso be noted that measurements can sometimes be strongly dependent orcorrelated (if for example the wavelength or angle of incidence AOI ischanged by a negligible amount, one cannot really gain new information,since the difference between measurements is lost in the measurementnoise). Therefore, the requirement for a high number of measurements isto be actually replaced by a metric such as the “condition number” usedin linear algebra. For example, considering the spatial distribution ofa material, the different field distributions probed at differentconfigurations (i.e. temperatures in this case) is to be sufficientlydifferent, i.e. orthogonal, to each other. Such orthogonality might bedifficult to achieve using only external beam parameters such as angleof incidence or numerical aperture, and very costly to achieve usingmany different exciting wavelengths. The present invention, utilizingmeasurement of the temperature profile of Raman scattering, provides arelatively cost-effective and robust solution to this problem.

FIG. 5 illustrates, in a self-explanatory manner, a flow diagram 200 ofa method of the invention using the above described technique todetermine parameter(s) of a structure being measured from spatialprofile of Raman scattering. The method utilizes known data (e.g.previously prepared and stored in database) about dependence of thestructure property/condition of interest (illumination penetrationdepth/absorption), as function of temperature and wavelength fordifferent materials, and one or more respective models of Ramanscattering—step 202. As described above, the model describes/providestheoretical data of Raman scattering within a parametric space of themodel, where the structure property/condition and the temperature areinclude in the model' parameters.

Measured data is provided (step 206) and processed (step 207), asdescribed above. The processing may be performed in an on-line mode, oroff-line mode using measured data previously obtained and stored. Themeasured data MD is a function of at least such parameters as wavelengthand temperature. For example, measurements may be applied to nmeasurement sites of a structure using m different temperatures—step204. The temperature data TD is determined, e.g. from the measured Ramanspectra, as described above (step 208), and used to optimize the Ramanscattering model (step 210). Then, the optimized model is used todetermine, via fitting procedure, the structure parameter(s)—step 212.

1. A method for use in measuring one or more characteristics of apatterned structure, the method comprising: performing measurements on apatterned structure by illuminating the patterned structure withexciting light to cause Raman scattering of one or more excited regionsof the patterned structure, while applying a controlled change of atemperature condition of the patterned structure and while monitoringthe temperature condition of the patterned structure; collecting theRaman scattering, by a detection unit, and generating measured data; andgenerating, utilizing the measured data, data indicative of spatialprofile of one or more properties of the patterned structure.
 2. Themethod according to claim 1, wherein said one or more properties of thepatterned structure include at least one of material and geometricproperties of the patterned structure.
 3. The method according to claim2, wherein said applying the controlled change of the temperaturecondition comprises controllably heating the patterned structure underillumination to affect corresponding change in optical absorption ofmaterials of the patterned structure within at least said one or moreexcited regions, thereby changing penetration of exciting illuminationinto the patterned structure, the measured data being thereforeindicative of a spatial profile of sources of the Raman scatteringwithin said one or more excited regions in the structure.
 4. The methodaccording to claim 3, wherein the regions of the patterned structure atdifferent penetration depths are different from one another in at leastone of the following: layer stacks and patterns.
 5. The method accordingto claim 1, wherein said controlled change of the temperature conditionis performed by heating of the patterned structure.
 6. The methodaccording to claim 1, comprising performing a plurality of themeasurement sessions while varying one or more of the followingmeasurement conditions: wavelengths of the illumination; angles ofincidence of the illumination; and polarization states of theillumination.
 7. The method according to claim 1, wherein saidillumination of the patterned structure to cause Raman scattering of oneor more excited regions of the pattern structure comprises optical beamsof two or more selected wavelengths.
 8. The method according to claim 1,wherein the measured data is a temperature dependent Raman response ofthe patterned structure, and wherein said monitoring of the temperaturecomprises analyzing the temperature dependent Raman response of thepatterned structure.
 9. The method according to claim 8, wherein saidspatial profile of the one or more properties of the patterned structureis a two-dimensional profile of said at least one property distributionacross and through the structure.
 10. The method according to claim 8,wherein said analyzing of the measured data comprises applying to saidmeasured data one or more predetermined models describing Ramanscattering from the structure, where the temperature condition and saidone or more properties of the patterned structure are included in a setof model parameters.
 11. The method according to claim 8 comprisingdetermining the change of the temperature condition from a variation ofa Raman scattering peak intensity.
 12. The method according to claim 8comprising determining the change of the temperature condition from avariation of a Raman scattering peak wavelength.
 13. The methodaccording to claim 8 comprising determining the change of thetemperature condition from a variation of a Raman scattering peak width.14. A system for measuring one or more characteristics of a patternedstructure, the system comprising: a control unit; and a measurementsystem that comprises an illumination unit and a detection unit; whereinthe illumination unit is configured and operable to illuminate thepatterned structure to cause Raman scattering of one or more excitedregions of the patterned structure and to cause a controllable change ofa temperature condition of the patterned structure; wherein thedetection unit comprises at least one optical detector configured andoperable to collect the Raman scattering of the structure while underthe controllable change of the temperature condition, and generatecorresponding measured data; and wherein the control system isconfigured to be in data communication with the measurement system, tomonitor the temperature condition of the patterned structure, to receivethe measured data, and to utilize the measured data to generate dataindicative of a spatial profile of at least one property of thepatterned structure.
 15. The system according to claim 14, wherein themeasured data is a temperature dependent Raman response of the patternedstructure, and wherein the control system is configured and operable tomonitor the temperature condition by analyzing the temperature dependentRaman response of the patterned structure.
 16. The system according toclaim 15, wherein the control system is configured and operable toperform the analyzing by applying to said measured data one or morepredetermined models describing Raman scattering from the structure,where the temperature condition and said one or more properties of thepatterned structure are included in a set of model parameters.
 17. Thesystem according to claim 15, wherein the control system is configuredand operable to determine the change of the temperature condition from avariation of a Raman scattering peak intensity.
 18. The system accordingto claim 15, wherein the control system is configured and operable todetermine the change of the temperature condition from a variation of aRaman scattering peak wavelength.
 19. The system according to claim 15,wherein the control system is configured and operable to determine thechange of the temperature condition from a variation of a Ramanscattering peak width.